Glicentin which is one of enteroglucagons is a peptide comprising 69 amino acid residues. For example, human glicentin is composed of the following amino acid sequence (SEQ ID NO:1)
Arg-Ser-heu-Gln-Asp-Thr-Glu-Glu-Lys-Ser-Arg-Ser-Phe-Ser-Ala-Ser-Gln-Ala- Asp-Pro-Leu-Ser-Asp-Pro-Asp-Gln-Met-Asn-Glu-Asp-Lys-Arg-His-Ser-Gln-Gly-Th r-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg-Ala-Gln-Asp-Phe-Val-G ln-Trp-Leu-Met-Asn-Thr-Lys-Arg-Asn-Arg-Asn-Asn-Ile-Ala
The sequence contains in positions 33-61 the structure of glucagon which is a peptide hormone comprising 29 amino acid residues. Glicentin and glucagon are produced by tissue specific processing from the same precursor, preproglucagon. Glucagon is formed in pancreas and glicentin in intestine. Regarding physiological roles of these peptides, glucagon is known to control glycogenolysis and blood glucose level. However, the role is unknown for glicentin.
Glucagon-producing tumors are a very rare disease. One case was reported by Gleeson et al. in Gut, 12, 773-782, 1971, which was characterized by symptoms in small intestinal structure and function such as thickened small intestinal villi, delayed passing rate of food through the digestive tract and constipation. These findings and symptoms were found to have disappeared after removal of the tumor by surgical operation, possibility for factors produced by the tumor to participate in the altered small intestinal structure and function was postulated. Later, it was discovered by Bloom in Gut, 13, 520-523, 1972 that the tumor produced enteroglucagon. It was thus suggested that such symptoms were due to biological activity of enteroglucagon. It was further observed in animal experiments that jejunectomy brought about an increase in blood enteroglucagon level and a promoted productivity of cryptocell, by which the proliferating effect of enteroglucagon on epithelial cells of the intestinal tract was supported (Brit. g. Surg., 69, 14-18, 1982). It is however uncertain whether enteroglucagon as a trophic factor brings about proliferation of the small intestinal mucosa or both the increase in blood level and the mucosal proliferation are due to other factors. In addition, it was reported that a crude extract of enteroglucagon stimulated DNA synthesis in cultured cells of the small intestine (O. Ottenthal et al. , Regul. Pept., 3, 84, 1982). However, it is uncertain whether the action is due to enteroglucagon itself, particularly glicentin, because its sample contains many impurities.
Enteroglucagon is also called gut glucagon-like immunoreactivity, which is defined on the basis of immunoreactivity against anti-glucagon antiserum of low specificity and contains a number of peptides produced by tissue specific processing from the same preproglucagon. A plural of candidates are presented as the active form in blood, including oxyntomodulin, i.e., a peptide hormone in positions 33-69 of the amino acid sequence of glicentin, glucagon 1-21, i.e., a peptide in positions 33-53, glucagon-like peptide-1, i.e., a peptide hormone which occurs in a region of preproglucagon containing no glicentin and similar glucagon-like peptide-2. Thus, the role of glicentin, a kind of enteroglucagon has not yet been elucidated and also its target tissue and cell are unclear. Moreover, human glicentin has neither been isolated nor studied as a substance.
The present inventors were successful in synthesizing DNA corresponding to the amino acid sequence of human glicentin which was deduced by G. I. Bell (Nature, 304, 368-371 (1983)) from the sequence of human preproglucagon gene and preparing human glicentin by means of genetic engineering procedure using the synthesized DNA (Japanese Patent Kokai Hei 4-364199). Thus, human glicentin has easily been made available in a large amount and as a purified product.
The intestinal tract is an organ mainly participating in digestion of food and absorption of nutritional elements. Ingestion of the nutritional elements necessary for life conservation is mostly effected via the intraluminal mucosal layer of the intestinal tract. It is therefore a serious problem to the living body to have the functions of the digestive tract impaired by histological atrophy, development of ulcers or reduction in function of the mucosa after pathological or surgical injuries, and further, to have permeability of the intestinal mucosa abnormally intensified to allow for translocation of bacteria or foreign bodies. Thus, when a digestive tract is invaded or atrophied, rapid cure and recovery of function of the digestive tract are desirable. In case of the hypoplasia of the digestive tract tissues, it is necessary to promote their growth and enhance their function.
It is also known that atrophy of the intestinal mucosa occurs by exposure to strong radiation or other cytostatic stimulus because turnover of the intestinal mucosal cells is very rapid. Furthermore, it is known that atrophy of the intestinal mocosa occurs also by adopting intravenous or parenteral nutrition which does not require intestinal functioning, or enteral nutrition or elemental diet which does not require normal digestive tract functions, when functions of the intestinal tract are reduced, or digestion and absorption in the intestinal tract fail after the intestinal tract operation. However, no drug is present which induces proliferation of the cells of intestinal mucosa for the treatment of functional disorder of the digestive tract caused by atrophy of the digestive tract mucosa. Therefore, there is a continuing desire to develop such drug.
Further, diseases associated with resection of digestive tract, e.g., damping syndrome associated with an extensive gastrectomy bring about abnormally accelerated excretion of the intragastric contents, thus leading to rapid migration of not completely digested food to the jejunum, which results in hypertonicity of the contents in the jejunum. This will induce various pathological conditions such as sweating, tachycardia and nausea. Short gut syndrome caused by extensive resection of the intestinal tract in the treatment of various diseases or reduction in function of the small intestine is accompanied by exaggerated secretion of gastric acid, exaggerated peristalsis of the stomach and other reactions. These reactions occur as a result of failure of the feedback mechanism due to lack of the cells in the digestive tract mucosa that produce digestive tract hormones, physiologically active peptides and the like.